The formula FeatureCAM uses is from the Machinery's Handbook Ed. 20 Pg. 1435:
drill size = outside dia * 1.299 * thread_percent * (1/threads_per_inch)
- drill_size is the size of the drill that is used to rough the hole.
- outside_dia is the size of the tapped hole, so for a "1/4-20", it would be 1/4in.
- thread_percent is Thread % for tap drill from the tool selection page of default attributes. By default it is 77%.
- threads_per_inch is the threads per inch. In the "1/4-20" example, it is 20.
The drill size table is based on 77% thread. Switch to 75% if you want the same behavior as the Machinery's Handbook.
Also, it should be noted that FeatureCAM's tool selection tolerance is 0.002 inches. This tight tolerance occasionally results in the failure to automatically select a tool. For example, for a 5/8” hole, the formula results in a number close to a standard drill size given in the handbook's table, but not within 0.002. Hence tool selection fails. In this case, you must explicitly select a tool.